Flip–flop kinetics, or flip–flop pharmacokinetics, describes an atypical situation in
pharmacokinetics
Pharmacokinetics (from Ancient Greek ''pharmakon'' "drug" and ''kinetikos'' "moving, putting in motion"; see chemical kinetics), sometimes abbreviated as PK, is a branch of pharmacology dedicated to determining the fate of substances administered ...
where a drug's rate of
absorption
Absorption may refer to:
Chemistry and biology
* Absorption (biology), digestion
**Absorption (small intestine)
*Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials
*Absorption (skin), a route by which ...
or the rate at which it enters the
bloodstream
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
is slower than its
elimination rate.
That is, when the ''k''
a (
absorption constant) is slower than ''k''
e (
elimination constant).
These circumstances can occur with
sustained-release
Modified-release dosage is a mechanism that (in contrast to immediate-release dose (biochemistry), dosage) delivers a drug with a delay after its route of administration, administration (delayed-release dosage) or for a prolonged period of time (e ...
formulations,
depot injection
A depot injection is a term for an injection formulation of a medication which releases slowly over time to permit less frequent administration of a medication. They are designed to increase medication adherence and consistency, especially in pa ...
s, and some
subcutaneous or
intradermal injection
Intradermal injection, often abbreviated ID, is a shallow or superficial injection of a substance into the dermis, which is located between the epidermis and the hypodermis. For certain substances, administration via an ID route can result in a ...
s.
Determination of Absorption Rate Constant
/ref> In the resulting slope of log plasma concentration (log Cp) versus time, the apparent ''k''e is determined by the ''k''a, and the apparent ''k''e is smaller than when the drug is administered intravenously or by immediate-release formulation. Depot injections such as depot antipsychotics and long-acting injectable steroid hormone
A steroid hormone is a steroid that acts as a hormone. Steroid hormones can be grouped into two classes: corticosteroids (typically made in the adrenal cortex, hence ''cortico-'') and sex steroids (typically made in the gonads or placenta). Wi ...
medications like estradiol valerate
Estradiol valerate (EV), sold for use by mouth under the brand name Progynova and Primiwal E4 and for use by injection under the brand names Delestrogen and Progynon Depot among others, is an estrogen medication. It is used in hormone therapy fo ...
, testosterone enanthate
Testosterone enanthate, sold under the brand names Delatestryl and Xyosted among others, is an androgen and anabolic steroid (AAS) medication which is used mainly in the treatment of low testosterone levels in men. It is also used in hormone th ...
, and medroxyprogesterone acetate
Medroxyprogesterone acetate (MPA), also known as depot medroxyprogesterone acetate (DMPA) in injectable form and sold under the brand name Depo-Provera among others, is a hormonal medication of the progestin type. It is used as a method of bi ...
are examples of drugs with flip–flop kinetics.
References
Pharmacokinetics
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